3.7.1 - Inheritance

Genes, alleles, genotype and phenotype

A pea plant with green pods may still carry the allele for yellow pods, and these terms explain how.

A gene is a DNA base sequence coding for a polypeptide or functional RNA. Alleles are different versions of a gene. They occupy the same locus (position) on homologous chromosomes, the matching pair inherited from the two parents.

So a diploid individual normally has two alleles at an autosomal locus (a position on any chromosome other than a sex chromosome), one on each homologue. A gamete has only one, because meiosis separates the homologous chromosomes. Follow one pair, $A$ and $a$, from left to right.

The two alleles separating into different gametes is called segregation. Each allele of the pair therefore has probability $\frac{1}{2}$ of entering a gamete, and fertilisation restores the pair.

Genotype and phenotype

For a cross, write only the alleles being considered, such as $Gg$. Genetically identical plants can differ in height if one receives less light, so a visible difference does not, by itself, prove a genetic difference.

In an exam, "the appearance of a characteristic" alone can earn nothing: link it to the genotype and the environment.

Dominant, recessive and codominant alleles

Homozygous means two identical alleles at a locus, such as $GG$ or $gg$; heterozygous means two different alleles, $Gg$. A heterozygote reveals how its alleles behave:

• Complete dominance: the dominant allele determines the heterozygote's phenotype. The recessive phenotype occurs only without a dominant allele at that locus.
• Codominance: both alleles are expressed in the heterozygote. Neither is recessive, so neither gets a lower-case letter; superscripts such as $C^R$ and $C^W$ show this.

Dominant does not mean common, beneficial, stronger or more likely to be inherited. It describes the heterozygote's phenotype only: in $Gg$, the $g$ allele is as likely to enter a gamete as $G$.

Multiple alleles means more than two allele versions occur in the population, though a diploid individual still has two at that locus.

Monohybrid crosses and test crosses

Follow one gene through two generations and the recessive phenotype disappears, then returns.

A genetic diagram tracks alleles from parental phenotype through genotype and gametes to offspring genotype and phenotype. It works in two directions: predict the offspring of a known cross, or interpret observed offspring to work out the parents. A Punnett square is only the fertilisation stage, not the whole explanation.

From pure-breeding parents to a segregating generation

A monohybrid cross follows one locus. A pure-breeding organism is homozygous at that locus, so it makes only one type of gamete. Bred with another like it, it gives offspring with the same feature.

Crossing two pure-breeding parents gives the $F_1$ (first filial generation). Crossing two $F_1$ plants gives the $F_2$ (second filial generation), called a segregating generation because the alleles separate again and both parental phenotypes reappear.

Read a cross backwards to find the dominant allele. If two parents with the same phenotype have an offspring with a different one, both carry an allele they do not show: both are heterozygous, and the hidden allele is recessive.

Parents sharing a recessive phenotype are homozygous, so they can pass on only that allele.

Turning a ratio into probabilities

A ratio is a set of probabilities. The 3 : 1 ratio has 4 parts, so the chance of green is $\frac{3}{4}$ and of yellow $\frac{1}{4}$. Each cell of the square is a probability of $\frac{1}{4}$, not a promise that four offspring include one of every cell.

To predict a count, multiply the probability by the total. Of Mendel's 580 $F_2$ pea plants, $\frac{3}{4}$ predicts 435 green; he counted 428 green and 152 yellow. Fertilisation is random, so real counts sit close to a ratio rather than exactly on it.

Successive fertilisations are independent in this model, which gives two rules:

• Multiply when one independent event and another must both happen. The probability of two yellow offspring is $\frac{1}{4}\times\frac{1}{4}=\frac{1}{16}$.
• Add when mutually exclusive routes lead to one outcome. Exactly one yellow in two offspring can come in either order: $\frac{1}{4}\times\frac{3}{4}+\frac{3}{4}\times\frac{1}{4}=\frac{3}{8}$.

Revealing an unknown genotype

A green plant could be $GG$ or $Gg$, and looking at it cannot say which. A test cross pairs an individual showing the dominant phenotype with a homozygous recessive individual. This tester can only supply $g$, which cannot mask anything, so each offspring's phenotype shows which allele it received from the unknown parent.

• If the unknown parent is $GG$, every offspring is green $Gg$.
• If it is $Gg$, its $G$ and $g$ gametes combine with the tester's $g$, predicting 1 green $Gg$ : 1 yellow $gg$.

A yellow offspring establishes that the unknown parent carried $g$. All-green offspring only support $GG$, because a small family could be all green by chance: six green offspring from the heterozygote test cross has probability $\left(\frac{1}{2}\right)^6=\frac{1}{64}$.

In an answer, name the cross, then state what each result would show. AQA's 2024 examiners saw students give the right cross but not explain how the offspring reveal the genotype.